1. Field of the Invention
The present invention relates to a document feeder for feeding documents one by one from a document placing portion to a document receiving portion via an image reading position, and more specifically to a document feeder for feeding not only paper documents having high flexibility but also small-sized card documents having low flexibility. The present invention also relates to an image reader and an image forming apparatus provided with such a document feeder.
2. Description of the Related Art
As a scanner for reading an image of a document, a document feeder for feeding documents one by one from a document placing portion to a document receiving portion via an image reading position is used. Such a document feeder as disclosed in Japanese Patent Laid-Open Publication No. 60-242762, for example, includes a freely movable pick-up roller in the upper part of an end portion on the downstream side of the document placing portion in a document feed direction, and a pair of feed rollers and a pair of conveyance roller (hereinafter referred to as a pair of registration rollers, as the case may be) between the pick-up roller and the document reading position.
The pick-up roller moves down to a sheet feeding position where the roller contacts with the upper surface of a document placed on a document platen while rotating in synchronization with the pair of feed rollers at start of feeding a document. The pair of feed rollers rotates while vertically holding the document therebetween. The pair of conveyance rollers starts rotating after a lapse of predetermined time from a time when the leading edge of the document reaches a document sensor disposed on the upstream side of the pair of conveyance rollers.
The document placed on the document platen is fed toward the pair of feed rollers by the rotation of the pick-up roller, and then guided to the pair of conveyance rollers by the rotation of the pair of feed rollers. The above-stated predetermined time is a time longer than the time required for the leading edge of a document sheet to reach the pair of conveyance rollers after having passed the document sensor. The fed position of the document is corrected by contacting with the leading edge thereof with the pair of conveyance rollers before the rollers start rotation.
The document feeder has a space for stacking and placing a plurality of documents above the document platen so that images on the plurality of documents are successively read. The pick-up roller moves up to a standby position which does not contact with the upper surface of the document placed on the document platen before starting feeding a document. After moving down to the sheet feeding position at the start of feeding a document, the pick-up roller moves up to the standby position when the leading edge of the document reaches the position of the document sensor.
Images on a plurality of documents being successively read, the pick-up roller moves up to the standby position every time when the leading edge of each document reaches the document sensor, and moves down to the sheet feeding position before feeding the following document. Such movement of the pick-up roller makes a larger feeding interval between documents, and lengthens operating time for feeding a plurality of documents.
In an attempt to overcome such problems, some conventional document feeders, in successively feeding a plurality of documents, cause a pick-up roller to move down to a sheet feeding position before starting feeding the first document, and cause the pick-up roller to move up to a standby position when the leading edge of the last document reaches a document sensor. A smaller feeding interval between documents can shorten time for feeding a plurality of documents.
In order to correctly read an image of a document in an image reading position, a document feed path between a feed roller and a document reading position is formed in a straight line, so that the nip portion of a pair of registration rollers is located on a line tangent to the nip portion of a pair of feed rollers. The placement surface of a document placing portion is located below a straight line connecting the feed rollers and the image reading position, and the end portion of the downstream side of the placement surface is composed of an inclined surface where the downstream side in the feed direction is located above the upstream side.
In the conventional document feeders, the pick-up roller also moves up and down in feeding a small-sized card document having low flexibility at the same timing as in feeding a paper document having high flexibility. In feeding a plurality of card documents placed on the document placing portion one by one, the pick-up roller keeps pressing the card document downward in a sheet feeding position until the leading edge of the last card document reaches a sensor position. The card documents other than the last card document are not guided between the pair of registration rollers because the leading edge located on the downstream side of the pair of feed rollers moves upward by being pressed downward on the upstream side of the pair of feed rollers.
An object of the present invention is to provide a document feeder capable of surely guiding the leading edge of a card document between a pair of registration rollers by causing a pick-up roller to move up to a standby position when the card document is conveyed at a timing different from when a paper document is conveyed, and an image reader and an image forming apparatus provided with the document feeder.